class_docstr: Data structure to store backdoor variables between 2 nodes.
code: def __init__(self, node1, node2):
      self._node1 = node1
      self._node2 = node2
      self._is_blocked = None  # To store if all paths between node1 and node2 are blocked
      self._condition_vars = (
          []
      )  # To store variable to be conditioned on to block all paths between node1 and node2
      self._complete = False
  
imports: import networkx
  from dowhy.utils.graph_operations import adjacency_matrix_to_adjacency_list
label_desc: Use image node_id 1 to create a new NodePair object  with example: obj = NodePair(node1, node2)